This invention is concerned with a tubular, perforating gun of the kind which is lowered on a cable through and out the open end of a string of drill tubing into an oil or gas well for the purpose of firing explosive charges to perforate hydrocarbon bearing subsurface strata, and in particular with the automatic separation of such perforating gun, upon its firing, from the firing assembly with which it was lowered into the well.
When perforation of an oil (or gas) well is to be accomplished by use of a perforating gun lowered into the borehole through and out the open end of the string of tubing (commonly referred to as "throughtubing"), a gun must be selected which has an outside diameter which is at least slightly smaller than the inside diameter of the tubing. If the diameter of the gun is too small, however, the size, and consequent power of the charges carried therein is limited resulting in adequate penetration of the formation.
In order to carry the most powerful possible charge, and thus to obtain the greatest possible penetration into the subsurface strata, guns have typically been constructed to maximum size, i.e., with the closest possible tolerance in terms of ability to pass through the string of tubing. The problem frequently encountered, being that which is addressed by this invention, is that, after firing, the gun is deformed. The power of the explosion expands the gun distorting the original size and shape of its tubular body, and burrs appear around the ports out of which the charges are fired where the force of the shock wave, upon its exit from the gun, has given the rim of the port a slight bend or turn toward the outside of the gun. This distortion in the shape of the gun, opposes smooth passage back through the string of tubing, and makes retrieval an extremely difficult task. If the gun becomes stuck in the tubing, which is frequently the case, because the original close tolerance is no longer sufficient, the cable upon which it was lowered down through the tubing may pull loose, and all operations on the well must cease while an effort is made to "fish" the lodged gun out of the string of tubing. As can be appreciated, the occurrence of such a situation adds enormous cost and inconvenience to the effort to put a well into production.
The concept of dropping a spent gun off into the rathole of the well is known in the art. For example, U.S. Pat. Nos. 3,966,236 and 4,066,282 disclose apparatus for releasing a spent perforating gun which has been lowered into the borehole by attachment directly to the end of the string of tubing, the release apparatus being activated by a fishing tool lowered down through the string of tubing on a wire.
The invention described herein not only furnishes novel method and apparatus for automatically releasing a spent gun which has been lowered on a cable into the borehole through and out the open end of the string of tubing, it does so using the explosive power of the perforating gun rather than by trying in some fashion to focus, regulate or ameliorate the exterior effect of such explosive power on the many different ways in which the gun may be attached to the assembly on which it is lowered through the tubing.